Thrombin is a member of the class of the serine proteases. Serine proteases are enzymes which, amongst other things, play an important role in the blood coagulation cascade. Other members of this class of proteases are for example trypsin, factors VIIa, IXa, Xa, XIa, XIIa, and protein C.
Thrombin is the serine protease which regulates the last step in the coagulation cascade. The prime function of thrombin is the cleavage of fibrinogen to generate fibrin monomers, which form an insoluble gel by cross-linking. In addition, thrombin regulates its own production by activating factors V and VIII earlier in the cascade. It also has important actions at the cellular level, where it acts on specific receptors to cause platelet aggregation, endothelial cell activation and fibroblast proliferation. Thus throinbin has a central regulatory role in haemostasis and thrombus formation. Since inhibitors of thrombin may have a wide range of therapeutical applications, extensive research has been performed in this area.
In the development of synthetic inhibitors of sernie proteases, and more specifically of thrombin, the interest in small synthetic peptides that are recognized by proteolytic enzymes in a manner similar to that of natural substrates, has increased. As a result, new peptide-like inhibitors have been prepared, such as the transition state inhibitors of thrombin.
The search for more effective and more selective thrombin inhibitors continues unabated in order to obtain thrombin inhibitors which can be administered in lower dosages and which have fewer and less severe side effects.
Furthermore, special attention is paid to oral bioavailability. Potent intravenous thrombin inhibitors are clinically effective in acute care settings requiring the treatment of thrombin-related diseases. However, particularly the prevention of thrombin-related diseases such as myocardial infarct, thrombosis and stroke require long-term therapy, preferably by orally dosing of an anticoagulant.
Many of the peptide-like thrombin inhibitors disclosed in prior publications are based on the sequence -D-Phe-Pro-Arg-, see for example compounds as disclosed by Brady et al. Bioorganic & Medical Chemistry, 3 (1995), 1063-78!. Thrombin inhibitors may also contain lysine side chains instead of arginine, Such as other inhibitors disclosed by Brady et al., and further by Jones et al. J. Enzyme Inhibition, 9 (1995), 43-60! and Lewis et al. Thrombosis and Haemostasis, 74(4) (1995), 1107-12!. Also thrombin inhibitors having an aminocyclohexyl moiety instead of 2 lysine or arginine side chain are known WO 94/25051!. From these and other references e.g. U.S. Pat. No. 5,523,308! further a number of variations at the C-terminus of these peptide-like thrombin inhibitors is known. The developments in this field have already improved the understanding of how to modulate the biological properties of this type of thrombin inhibitor. However, although great effort is being spent on finding selective thrombin inhibitors having good oral bioavailability, there are still few transition state thrombin inhibitors known in the art which fulfill these requirements.